A light bulb attaches to a fitting in one of two ways. It either screws in on a threaded base, or it pushes in against a spring and twists a quarter turn, locking two pins into slots.
Both systems are over a century old. Both are still in large-scale production. Neither has displaced the other, and which one is in your ceiling depends almost entirely on where you happen to live.
This is unusual. Most competing standards resolve. This one did not, and the reason is that both designs are genuinely good at different things.
What Each One Is Actually Doing
The screw cap is a threaded metal shell that is itself one of the two electrical contacts, with the second contact as a small pad at the very bottom of the base. Screwing the bulb in simultaneously holds it mechanically and makes both connections.
The bayonet cap has two pins on the sides of a smooth cylindrical base. The bulb pushes in against a spring, the pins enter L-shaped slots, and a quarter turn seats them under a lip. The spring pushes the bulb outward against that lip, which both holds it in place and maintains pressure on two contacts at the base.
The difference that matters is how each resists vibration. A threaded joint can unscrew, slowly, if it is shaken – and a loosening bulb makes intermittent contact, which arcs, which damages the contact. A bayonet cap cannot work loose from vibration alone, because escaping requires a specific push-and-twist that shaking does not supply.
In exchange, the screw thread allows the bulb to be seated at any depth and tightened to any tension, which makes it tolerant of manufacturing variation in a way the bayonet’s fixed geometry is not.
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The Safety Arguments Point Both Ways
The most common claim is that one system is safer than the other, and this is more balanced than people usually allow.
Against the screw cap: the threaded shell is a large exposed conductor, and as the bulb is unscrewed it remains live while becoming accessible, so there is a moment during removal when a finger can reach a live surface. Correct wiring puts the neutral on the shell and the live on the bottom contact, which largely addresses it, and a reversed connection reinstates the hazard entirely.
For the screw cap: that same simplicity makes it easy to manufacture to a reliable standard, and the deep thread holds a heavy bulb well.
Against the bayonet: both contacts are at the base and the pins can, in a damaged fitting, be awkward to engage correctly.
For the bayonet: it cannot be partially inserted and left live and exposed in the same way, it resists vibration, and the quarter turn makes correct seating positive and obvious.
Neither is dangerous when properly made and properly wired. The honest summary is that the two have different failure modes rather than different safety levels, and the energy spent arguing about it considerably exceeds the difference.
Why the Split Is Geographical
The division is not technical. It is historical, and it followed electrification.
Wherever a system became established, it became the fitting in millions of ceilings, which made it the fitting lamps were sold with, which made it the fitting the next generation of buildings used. Electrical fittings are the definition of an installed base: changing one means changing every fitting in every building, and no benefit available from either design is remotely worth that.
So the pattern you see today is a map of which standard reached which place first and then locked in. Where both arrived in a similar period, both persist side by side, and houses contain a mixture.
It is one of the clearest everyday examples of path dependence – an outcome determined not by which option is better but by the order in which things happened, and then frozen by the cost of changing.
The LED Changed the Argument Completely
All of the above was developed for filament bulbs, and filament bulbs are gone from general lighting. That changes the terms substantially.
Filament bulbs failed frequently, which meant the cap was an interface used over and over throughout the life of a fitting. Ease and safety of repeated replacement was therefore a genuine design consideration.
An LED lamp lasts long enough that it may be replaced only a handful of times in the life of a room, and integrated LED fittings are not replaced at all – the light source is built in and the whole unit is changed when it fails, which is a different and considerably worse proposition for repair and waste.
So the cap is becoming less important as a point of interaction and more important as a legacy compatibility requirement. The reason LED lamps are still made in both cap types is not that either is well suited to an LED. It is that there are hundreds of millions of existing fittings and a lamp that does not fit them is unsellable.
Which means the shape of the thing you screw or push into your ceiling is now determined entirely by decisions made for a technology that no longer exists.
Both Systems Came From the Same Problem
It is worth knowing what either design was solving, because it was not obvious at the time that a bulb should be removable at all.
Early electric lighting had lamps wired or clamped in place, which was acceptable when installations were few and were maintained by the people who built them. As lighting spread into ordinary buildings, the filament’s short life made that untenable – a lamp that fails every few hundred hours cannot require an electrician.
So the requirement was a connection that an untrained person could make and break repeatedly, in poor light, above their head, at arm’s length, without tools, getting it right every time, with live conductors a short distance away.
Stated that way, the achievement is considerable. Both solutions are operable one-handed, give unambiguous feedback that they have worked, are impossible to assemble in a wrong orientation that would function incorrectly, and have remained unchanged for over a century while everything they connect to has been replaced several times over.
They also both solved it before most of the people using them had grown up with electricity at all, which is to say the designs had to be learnable in one demonstration by somebody with no prior concept of what they were doing. That is a harder design brief than almost anything in a modern house, and two separate answers to it both turned out to be good enough to survive indefinitely.
The Sizes Are a Separate Mess
Within each system there are multiple sizes, and the naming is not intuitive.
Screw caps are designated by a letter and a number giving the diameter of the base in millimetres, so the number is a measurement rather than a model. Bayonet caps are designated similarly, with variants for the small sizes and for the arrangement of the pins – including an offset-pin version used where it matters that a lamp can only be inserted one way round.
That last detail is a nice piece of design. Making the pins asymmetric turns a fitting into something that physically rejects the wrong lamp, which is the same principle as a plug that only goes in one way. It is used where polarity or lamp type must be guaranteed, and it costs nothing to implement because the slot geometry was already there.
The practical consequence for anybody buying a bulb is that the system – screw or bayonet – is the easy part, and the size is where people go wrong. Taking the old bulb to the shop remains, after a century of standardisation, the most reliable method available.
The other thing worth checking before you buy is the shape of the glass rather than the base, because a lamp that fits electrically can still be the wrong physical size for a shade or an enclosed fitting. Enclosed fittings matter more with LEDs than they did with filament lamps: the electronics in the base need to shed heat, and a lamp not rated for an enclosed fixture will run hot and fail early in one. That rating is printed on the box and is the single most ignored piece of information on it.
Have a look at what is in your own ceiling. If your house is old enough you may well have both, in different rooms, fitted by different people in different decades.
Neither of them won, and neither of them is going to.
Which makes the humble bulb cap a small monument to something worth noticing: that a standard does not have to be the best one to become permanent. It only has to arrive first, get installed in enough ceilings, and make replacement cost more than tolerance. Almost everything in a building works this way, and the light fitting is simply the one you handle often enough to notice.
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